1. Field of the Invention
The present invention relates in general to resonance tomography (MRT), as IS applicable in medicine for the examination of patients. The present invention relates in particular to an MRT method in which the image quality is improved in the case of images with spectral saturation or excitation.
2. Description of the Prior Art
Magnetic resonance tomography is a tomographic imaging modality for medical diagnostics, which is characterized first and foremost by a high contrast resolution capability. On the basis of an outstanding ability to represent soft tissue, magnetic resonance tomography has developed to a method far superior to X-ray computed tomography. Magnetic resonance tomography today is based on the application of spin echo and gradient echo sequences, which in the case of measuring times of the order of minutes makes an excellent image quality possible.
The presence of both fat and water tissue in a patient constitutes one challenge in magnetic resonance imaging. Due to the phenomenon known as the chemical shift, artifacts arise at the boundary layers between fatty tissue and aqueous tissue, which it is necessary to eliminate. Additionally, the fat signal often reduces the distinguishability of details in MR images both in T1-imaging and in T2 imaging because of its high signal contribution to the overall MR signal.
Chemical shift refers to the fact that the resonant frequency of the MR signal emitted by a nucleus will shift slightly in proportion to the magnetic field in which the nucleus is located, dependent on the type of chemical bond in which the nucleus participates. On the basis of their concentration in the human body, mainly hydrogen nuclei in free water and in fat contribute to the image. Their relative resonance frequency difference amounts to about 3 ppm (parts per million). As a result, in the case of the use of spin echo as well as gradient echo sequences, there is a modulation of the signal intensity that is dependent on the echo time TE.
According to the state of the art there are currently three basic methods for eliminating the disturbing fat signal:
1) The so-called STIR technology (STIR: Short Inversion Recovery),
2) Phase methods, which use the so-called 1-, 2- or 3-Punkt-Dixon-Verfahren, as well as
3) The spectral saturation method.
STIR is an IR sequence (Inversion Recovery sequence) with short inversion time. In the case of IR sequences the longitudinal magnetization is first inverted by a 180°-Puls in the opposite direction.
At the time when all fat protons are in a magnetization state Mz=0, a 90° excitation pulse is applied. Since the fat protons do not contribute to the resulting overall signal, the fat on the images obtained in this manner is suppressed and with this appears black.
Along with the relatively long acquisition time the disadvantage of STIR is among other things the relatively low overall signal yield (poor signal to noise ratio with low anatomical detail of the image). Another disadvantage is the fact that STIR cannot be used for contrast agent based MRT measurements.
Dixon methods are MR techniques for fat-water separation, which utilize the different resonant frequencies of fat and water protons (the chemical shift itself). Essentially in-phase images and opposite-phase images are acquired, and through whose mutual addition and/or subtraction pure water images and/or pure fat images can be generated.
However, the 2-point Dixon method fails in the case of voxels, in which the signals of fat and water in cancel each other out, which thus contain approximately equal amounts of water and fat. In such a case the phase between fat and water disappears in the signal noise.
For various reasons (shorter measuring time, better contrasts, less expensive post-processing) spectral (fat and water respectively) saturations and (water and fat respectively) excitation methods have advantages vis-à-vis the methods according to 1) and 2). The present invention is concerned with improving spectral saturations and excitation methods.
In the case of spectral fat saturation methods prior to every k-space measurement in the frequency encoding direction (measurement of a k-space line) spectrally selective RF-excitation pulse is emitted, which excites only the fat, and the longitudinal magnetization in the fat is converted to a transverse magnetization. This in turn is immediately dephased with the use of a magnetic field gradient. The directly following excitation pulse of the imaging sequence then no longer finds any convertible longitudinal magnetization and the fatty tissue thus does not get represented.
A complete and consistent fat saturation requires at least one homogenous magnetic field (B0 field) over a majority of the FOV's of the respective sub-coils of the RF (radio-frequency) antenna arrangement, so that the two lines (fat and water) in the spectrum can be cleanly separated. In the presence of ferromagnetic or metallic objects (implants, buttons, jewelry) or in the case of a wide variety of tissue types with different magnetic susceptibility regions (neck, thorax, knee) there are significant local magnetic field inhomogeneities, which ultimately leads to a local variability of the resonant frequencies.
One possibility for adjusting the homogeneity of the magnetic field is called “shimming”. For this reason most MRT systems have so-called shim coils, which are able to compensate to a great extent even more complex spatial progressions of magnetic field inhomogeneities.
However in the regions of the cervical vertebral column that have already been mentioned as well as in the knee, the many different tissue volumes change over a few centimeters so starkly that, due to the changing susceptibility values associated with this (tissue to air approximately 6 ppm), the B0 field variation with respect to the overall image cannot be compensated in spite of the shimming.
If in this case the frequency adjustment measurement necessary for shimming and for setting the device to the resonant frequency is carried out prior to the actual MR measurement, a fat-water spectrum is obtained that is composed of offset lines (doublet spectrums) of the different image regions. By a summation of the spectra, from a number of clean doublet spectra, one unclean triplet spectrum is brought about by the shift, and due to this triplet spectrum the resonant frequency (e.g. W0 for water) no longer is able to be exactly determined.
Within the scope of the necessary frequency adjustment (also after preceding shimming) the resonant frequency can then only be set with insufficient precision from the measured superimposed lines of the spectrum, which in individual critical partial images leads to undesirable saturation effects.